Motor Starting - TM-55-1930-209-14P-9-4

Fig. 1. Block Diagram of Excitation System

TM-55-1930-209-14P-9-4 Water Purification Barges (NSN 1930-01-234-2165) Electric Power System Manual
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Motor Starting An overload capacity equivalent to 250% full load impedance at zero p.f. can be sustained for 10 seconds. Short Circuit A current forcing unit must be fitted at the time of manufacture to provide a sustained short circuit current for discrimination of protective devices. Following the initial short circuit current envelope determined by the machine reactances/time constants, the standard machine does not support a sustained short circuit current. Voltage Build Up The exciter design is such that voltage build-up is achieved from residual levels only. Standby Ratings These are in accordance with NEMA MG 1 - 22.84. Parallel Operation (SC Machines Only) The standard machine is fitted with a fully interconnected damper winding and the standard phases sequence is U-V-W when rotating clockwise viewed from the drive end. When paralleling Stamford a.c. generators the neutrals can be connected. When paralleling dissimilar machines it is recommended that the neutrals are not connected as differences in waveform may result in harmonic currents. For types of parallel operation refer to Section - Six Accessories. AUTOMATIC VOLTAGE REGULATOR TYPES PC (Series 6) The A.V.R. on this frame designation is two phase sensed and controls the voltage within close limits under all load conditions providing high performance and the flexibility needed for most generator applications. The external wiring has been kept simple and comprises 3 push-on-type connections. By keeping the wiring simple no optional electrical accessories are available. SC (Series 4) Two types of A.V.R. can be fitted on this frame designation depending upon the application and performance required:- Type 1. The standard A.V.R. is two phase sensed and controls the voltage within close limits under all load conditions, providing high performance and flexibility needed for most generator applications. The design is such that a wide range of electrical accessories can be added to enhance the Specification. Type 2. This comprises a special A.V.R. and three phase sensing unit for special applications and derives its sensing voltage from all 3 phases of the machine. All generators when an unbalanced load is applied, will result in differences between phase to phase and phase to neutral voltages depending upon the degree of unbalanced load. It is normally accepted that if unbalanced loading can be limited to within 30% the impact on the machine regulation will be marginal on both two phase and three phase sensed A.V.R.'s. If, for any reason, unbalanced loading occurs above this level the overall regulation will exceed the normal specified limits. On a two phase sensed A.V.R., the phase directly linked with the A.V.R. sensing, will be maintained to close regulating limits but the other phase will float outside the normal regulation limits. By incorporating a three phase sensed A.V.R. overall regulation will not improve but the overall balance around a set point will improve. For example, if the overall voltage regulation is 5% on a two phase sensed A.V.R. the regulation could be all one way either +5% or -5% on one of the phases and close to 0% on the other phases. On the three phase sensed A.V.R. although the overall regulation would still be 5% this would tend to float around the nominal setting to give a regulation of +2% between all phases. A second advantage of three phase sensing is that it provides a more stable reference when operating on thyristor or rectifier loads which, due to the high harmonic content in the current waveform distort the generator waveform. The recommendation is that where high thyrsitor or rectifier loads are used, the best results will be obtained with a three phase sensed A.V.R. NOTE: A range of electrical accessories is available for both two phase and three phase sensed A.V.R.'s see Section 6 for details. MARINE MACHINES (SC TYPE ONLY) In order to meet the requirements of marine classifying societies certain modifications to the standard machine are required. From the electrical performance point of view the standard machine is suitable for marine use providing a current forcing unit is fitted to give short circuit maintenance for discrimination of protective devices. From the mechanical point of view it is necessary to incorporate a shaft manufactured from approved material and to add drip-proofing features to cater for the inclination of the machine in the form of louvered covers on the air outlets The requirements of most main classifying societies can be met in this way. 8

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